Document FR-A-2 253 723 describes a method of preparing a preform for an optical waveguide in which a plasma torch projects glass powder which is injected into the base of the flame by a carrier gas. A flow of carrier gas is created so as to move the glass powder along a substantially horizontal elongate path. More precisely, in the absence of carrier gas, the glass powder does not advance towards the plasma torch so the plasma flame does not contain glass powder. That constitutes a method of pneumatically injecting glass powder.
Also known, from document GB-A-2 134 696 is a method of manufacturing a preform which is similar to the method of the first-mentioned document. A mechanically measured quantity of silica glass powder is inserted into a central tube into which there are inserted in succession oxygen and hydrogen to form an oxygen-hydrogen torch flame, so that the powder is inserted into the torch before a flame is formed. That method therefore includes injecting glass powder upstream from the base of the flame and via the inside of the flame.
Document FR-A-2 446 264 describes a method of preparing a preform for an optical waveguide in which a circular section cylindrical preform is moved along its axis past the flame of a plasma torch, said flame extending horizontally. A feed duct having a sloping bottom length inserts silica glass powder under the action of gravity to the top of the flame so that the powder is entrained by the flame onto the preform. However, although that document indicates that the plasma torch can be oriented in several ways in a vertical plane perpendicular to the axis of rotation of the preform, the description states that the grains of glass "fall" into the flame of the plasma, which means that the plasma flame must have a significant component in a horizontal direction.
The disposition described in that document FR-A-2 446 264 is simpler than the dispositions described in the previous documents because the operations of conveying the glass powder and of forming the plasma flame are completely separated and because glass powder is fed solely under the action of gravity.
In general terms, the invention relates to a method of the type described in above-mentioned document FR-A-2 446 264 in that it implements a plasma flame that is directed towards a preform, together with a separate device for feeding powder under gravity. Nevertheless, in the method of the invention, the glass powder gravity feed normally takes place only at the edge of the plasma flame. In such a method, the quantity of glass powder inserted into the plasma and deposited on the primary preform is limited by the depth to which the powder penetrates into the plasma flame.